APOPTOSIS OF THE MIDLINE GLIA DURING DROSOPHILA EMBRYOGENESIS - A CORRELATION WITH AXON CONTACT

We have examined cell death within lineages in the midline of Drosophi la embryos. Approximately 50% of cells within the anterior, middle and posterior midline glial (MGA, MGM and MGP) lineages died by apoptosis after separation of the commissural axon tracts. Glial apoptosis is b locked in embryos deficient for reaper, where greater than wild-type n umbers of midline glia (MG) are present after stage 12. Quantitative s tudies revealed that MG death followed a consistent temporal pattern d uring embryogenesis. Apoptotic MG were expelled from the central nervo us system and were subsequently engulfed by phagocytic haemocytes. MGA and MGM survival was apparently dependent upon proper axonal contact. In embryos mutant for the commissureless gene, a decrease in axon-gli a contact correlated with a decrease in MGA and MGM survival and accel erated the time course of MG death, In embryos mutant for the slit gen e, MGA and MGM maintained contact with longitudinally and contralatera lly projecting axons and MG survival was comparable to that in wild-ty pe embryos. The initial number of MG within individual ventral nerve c ord segments was increased by ectopic expression of the rhomboid gene, without changing axon number. Extra MGA and MGM were eliminated from the ventral nerve cord by apoptosis to restore wild-type numbers of mi dline glia. Ectopic rhomboid expression also shifted MGA and MGM cell death to an earlier stage of embryogenesis. One possible explanation i s that axon-glia contact or communication promotes survival of the MG and that MG death may result from a competition for available axon con tact.